linux-kernel - Re: [PATCH v4 2/6] dma-buf/heaps: Don't assert held reservation lock for dma-buf mmapping

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Message-ID: <106bc72b-9358-de06-6f3f-06fdfa3859cd@collabora.com>
Date:   Wed, 21 Jun 2023 21:16:29 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     "T.J. Mercier" <tjmercier@...gle.com>
Cc:     Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Benjamin Gaignard <benjamin.gaignard@...labora.com>,
        Brian Starkey <Brian.Starkey@....com>,
        John Stultz <jstultz@...gle.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Tomi Valkeinen <tomba@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Emil Velikov <emil.l.velikov@...il.com>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
        kernel@...labora.com, linux-media@...r.kernel.org
Subject: Re: [PATCH v4 2/6] dma-buf/heaps: Don't assert held reservation lock
 for dma-buf mmapping

Hi,

On 6/21/23 20:21, T.J. Mercier wrote:
> On Mon, May 29, 2023 at 3:46 PM Dmitry Osipenko
> <dmitry.osipenko@...labora.com> wrote:
>>
>> Don't assert held dma-buf reservation lock on memory mapping of exported
>> buffer.
>>
>> We're going to change dma-buf mmap() locking policy such that exporters
>> will have to handle the lock. The previous locking policy caused deadlock
>> problem for DRM drivers in a case of self-imported dma-bufs once these
>> drivers are moved to use reservation lock universally. The problem
>> solved by moving the lock down to exporters. This patch prepares dma-buf
>> heaps for the locking policy update.
>>
> Hi Dmitry,
> 
> I see that in patch 6 of this series calls to
> dma_resv_lock/dma_resv_unlock have been added to the
> drm_gem_shmem_helper functions and some exporters. But I'm curious why
> no dma_resv_lock/dma_resv_unlock calls were added to these two dma-buf
> heap exporters for mmap?

DMA-buf heaps are exporters, drm_gem_shmem_helper is importer. Locking
rules are different for importers and exporters.

DMA-heaps use own locking, they can be moved to resv lock in the future.

DMA-heaps don't protect internal data in theirs mmap() implementations,
nothing to protect there.

-- 
Best regards,
Dmitry